Precision resistors are in general use in Si-based microelectronics integrated circuit chips. These resistors are frequently fabricated from polysilicon layers deposited on the chip, but they can also be made from diffused silicon (Si) layers in silicon-on-insulator (SOI) wafers, or from refractory metal layers such as TaN or TiN. The resistor materials are generally high in electrical resistivity, and their total resistance is controlled by the film thickness and the width and length of the rectangular film segment that is used. The resistance is given by R=rho*l/A, where rho is the electrical resistivity, l is the length (direction parallel to the current flow) of the rectangle, and A is the cross-sectional area (thickness times width). In the case of Si resistors, the resistivity can be tailored by implantation of dopants, which increase the conductivity.
All of these resistor types, however, produce heat when current flows through them. The heat that is generated in the resistor increases the resistivity through the thermal coefficient of resistance (β). The generated heat can also permanently alter the value of the resistance by changing the grain size of the polysilicon, by burning out portions (or all) of the film and by redistributing the dopant atoms. These effects limit the amount of current that the resistor can tolerate. In addition to effects on the resistor itself, the generated heat may be conducted into the metal lines that are connected to the resistor and also into metal lines that may be located immediately above the resistor. Heating of the attached and nearby metal structures increases the susceptibility of the metal to electromigration, a process that produces holes in the metallization in response to current flow. Consequently, limiting the current through the resistor protects both the resistor stability and the integrity of the nearby metallization.
However, limiting the current through a resistor is at odds with the continued drive toward circuit miniaturization and the trend toward progressively greater current densities for high-performance circuits. That is, the heating constraint on resistor current is contending with circuit miniaturization and with circuit power requirements.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.